The present invention relates to an apparatus applied to an optical disc drive with adjustable focus offset and a method thereof.
When an optical disc is accessed by a pickup head in an optical disc drive, the conventional servo system of the optical disc drive will control the pickup head to emit a controlled laser emission to a proper location on the optical disc. The optimum focus point of the laser power (emission) upon the optical disc varies with the type of pickup head, optical disc, etc. This is because different type of pickup heads will have different structure of optical paths, and different types of optical discs will have different thicknesses. In other words, the minimum focus error (FE) signal will not guarantee that the current focus point is the optimum focus point upon a specific optical disc. Therefore, in the conventional focus control loop, a focus offset is added to the focus error signal to generate an adjusted focus error signal, which then the adjusted focus error signal is transferred to a controller. The controller tunes the position of the pickup head to optimize the focus of the laser spot upon a data recording layer of the optical disc. Accordingly, an optimized focus offset will be obtained to achieve an optimum reflected signal quality.
Furthermore, it is well-known that higher rotational speeds of the optical disc result in decreased SNR performance, degrading the decoding accuracy of the physical address and data of the optical disc. Thus, to increase the reading quality of the optical disc, strength of the laser power is dependent on the tangential velocity of the position of the optical disc to be accessed. In other words, the focus offset should be adjusted when the laser power changes. If the same focus error is adopted and added to the focus error signal in a case where the laser power changes in response to the tangent velocity change, the actual focus of the laser spot is shifted from the optimum focus. Therefore, a novel scheme of dynamically calibrating the focus error is required to optimize the focus of the laser spot for every position of the optical disc to be accessed.